1. Field of the Invention
The present invention relates to a coherent growth substrate. The present invention relates more particularly to a coherent growth substrate which contains a specific quantity of clay and a moisture content related to the specific matrix and clay at a specific suction pressure.
2. Description of the Prior Art
From DE-A-3 121 277 is known a growth substrate for plants which consists of a non-coherent mixture of burnt and expanded clay and of mineral fibres.
EP-A-0 350 132 describes a coherent growth substrate on a basis of mineral wool and clay which contains more than 50% by weight of clay, has a density of more than 200 kg/m.sup.3 and further has a moisture content of at least 15% by volume at pF=2.
The present invention is based on the insight that with smaller quantities of clay and at a lower moisture content at pF=2 a coherent growth substrate can be provided which enables an optimum growth development and results in a considerable increase in the product yield.
The added quantity of clay is not so much intended to increase the moisture content at a suction pressure of pF=2, but is found to have a signalling function whereby a temporary decrease in the moisture content below a minimum value has no adverse effects on the plant. The clay which is present evidently provides a signal to the plant of an imminent too low moisture content.
In addition, the smaller quantity of clay nevertheless provides a sufficient cation-exchanging complex for adsorption of main and trace elements, whereby in sub-optimal conditions a supplementation of these elements to the nutrient solution in the substrate is possible.
This small quantity of clay further has an already optimal effect on the developing microflora and fauna in the growth substrate during culture.
Lastly, this smaller quantity of clay already functions as an adsorbent for "phyto" toxic substances.
The coherent growth substrate based on a synthetic matrix according to the invention is therefore characterized in that it contains clay in a quantity of 0.1-25% by weight such that at a suction pressure of pF=2 the moisture content is lower than 15% by volume.
The improvement in the plant growth and product yield is considerable and amounts, depending on the crop, the matrix and the culture conditions, to 5-20% by weight, wherein an average yield improvement of up to 15% is possible. The improved product yield is found to be realized particularly when the plant culture proceeds for a longer period under less than optimal conditions.